1. Field of the Invention
This invention relates to aircraft propulsion systems and more specifically, to an aircraft propulsor system having counterrotating, highly swept, wide chord, very thin propulsor blades which are constructed primarily of composite materials.
2. Background Discussion
The basic aircraft propeller normally includes two or more blades connected to a central hub being driven by an aircraft power plant. The propeller pulls the airplane through the air by generating thrust obtained by the action of the rotating blades on the air. The propeller is generally described by reference to the leading edge (the first edge to cut into the air), the trailing edge (the last edge to contact the air), the front side or face, and the back or chambered side.
The propeller system contemplated in this invention is a counterrotating propeller system having a fore propeller with five to fifteen blades and a counterrotating aft propeller with from five to fifteen blades.
Previous propeller blade designs were adequate for low speed flight. However, numerous structural problems causing decreased performance result when these blades are used at high (near supersonic) rotational velocities. The structural problems for a blade operating at a very high speed result from the centrifugal and air turbulence forces and stresses acting on the blade.
One force acting on a blade in flight is a thrust force caused by air reacting against the blade parallel to the direction of advance. This thrust force produces a bending stress or torque in the blade. Another force is the centrifugal force caused by the rotation of the propeller tending to throw the blade radially outward from the axis of rotation. Centrifugal force produces tensile stresses in the blade. Another force acting on the blades is a torsional force caused by the air flow along edges of the blade producing a twisting force on the blade. This torsional force produces torsional stress in the blade. Thus, the primary stresses acting on a blade rotating at high speeds are bending stresses, tensile stresses, and torsional stresses.
The bending stresses bend the blade forward as the airplane is moved through the air by the propeller. Tensile stresses stretch the blade. Torsional stresses twist the blade. Additionally, torsional stresses are produced in rotating blades by two twisting moments, i.e., the aerodynamic twisting momement and the centrifugal twisting moment. The air reaction on the blade causes the aerodynamic twisting moment and the centrifugal force causes the centrifugal twisting moment. During ordinary propeller operation, the torsional forces tend to twist the blade to a lower blade angle resulting in blade inefficiency. Additionally, air turbulence generated by a fore propulsor in a counterrotating propeller system creates additional forces and stresses on the aft propulsor. In addition to the requirements for normal operation, the blade must be able to withstand impact with foreign objects such as birds and gravel.
High speed propulsors must be capable of withstanding aditional stresses at very high propulsor tip speeds. Then the tip of a propeller blade travels at a rate of speed which approaches the speed of sound (i.e., Mach 1.0), flutter or vibration causes other stresses to develop. If only a section of the blade exceeds the speed of sound, a shock wave can be generated and drastically decrease blade performance.
One method of overcoming the shock wave problem is by sweeping the leading and trailing edges of the blade so that the net airflow vector is less than Mach 1.0 even at high speeds. Sweeping the blade indicates a bending of the blade axially with respect to the aircraft direction of travel so that the leading edge trails behind a radially inward section of the leading edge and so that the trailing edge trails behind a radially inward region of the trailing edge. For example, U.S. Pat. No. 3,989,406 describes a swept blade for reducing leading edge shock in transonic and supersonic rotor blades in turbofan engines by sweeping the leading edge. Basically, for a swept blade the airspeed vector is the sum of the perpendicular airspeed vector and the tangential vector. The tangential vector is ignored for most purposes. Therefore, blade sweeping decreases the net airspeed vector below supersonic speed.
A structural solution to the blade stress problem has been the development of fiber reinforced resin-bonded structural composite materials. These materials have created a new design flexibility for propellers. There are three major advantages to the application of fiber reinforced composites. First, complex airfoil configurations can be shaped. Second, composite materials creates weight savings. Third, the dynamic frequencies and structural responses of the blade element can be tailored to its operating parameters. The present invention overcomes the problems and disadvantages of the prior art by providing a swept propeller blade comprised of composite materials having the strength and airfoil configuration to provide an efficient blade for a counterrotating propeller system.